나다 is used when something happens on its own..
내다 is used when something was made to happen
일이 끝났다 The job ended.
일을 끝냈다 The job was finished. (someone made it end)
땀이 났다 Someone perspired
땀을 냈다 Someone made perspiration happen.
돈이 났다 Money appeared... someone came upon some money.
돈을 냈다 Someone paid (for something)
열이 났다 Someone had a fever (it happened on its own).
열을 냈다 Something made something hot, gave off heat, got worked up (heated/ angry)
As Pascal mentioned, pay attention to the particles 을/ 를.